The liquid crystal display device, which has now come to be largely employed in the small size display device such as, for example, a mobile phone, a PDA or a video camera, or the medium-to-large size display device such as, for example, an automobile navigation system, a laptop computer, a computer display, a television or a billboard is generally made up of a planar light source element (a back light source) for emitting rays of light in a planar configuration, in combination with a liquid crystal display panel for providing a visual presentation of information. The liquid crystal display device is so designed that images of alphanumerical characters and/or pictures can be displayed by selectively controlling the light transmissivity of the liquid crystal display panel in response to applied audio/video signals.
The structure of the standard planar light source element 10 is shown in FIG. 3. Rays of light emitted from a light source 11 such as, for example, a cold cathode flourescent lamp or a light emitting diode (LED) income upon and then propagate within a light guide plate 12, made of a transparent material such as, for example, a polymethyl methacrylate or a cycloolefinic polymer. The light guide plate 12 has a rear surface provided with a rear surface treated pattern 13 including, for example, a pattern of dot or a micro-pattern. The rays of light propagating within the light guide plate 12 and subsequently incoming upon the rear surface treated pattern 13 are diffused and emerge outwardly from the light guide plate 12. Since the rays of light emerging outwardly from the light guide plate 12 are omnidirectional, the use is generally made of both a light diffusion film 14 and a plurality of prism sheets 15 so that the rays of light can be collimated to travel forwardly with an increased brightness. As shown in FIG. 3 the standard planar light source element includes one light diffusion film 14 and two prism sheets 15 having respective lenslets laid perpendicular to each other. The planar light source element also includes a light reflecting sheet 16 such as, for example, a film or a foamed polyethylene terephthalate, having a metal vapor deposited thereon, which is disposed adjacent to the rear surface of the light guide plate 12. Thereby, the rays of light emitted rearwardly of the light guide plate 12 can be returned to the light guide plate 12 by such a reflecting sheet 16.
The light diffusion film of the planar light source element of the structure referred to above does not only collimate omnidirectional rays of light in a direction forwardly thereof, but also serves to improve the uniformity of the planar light distribution, as well as to suppress glare of the planar light source element, that is, garish shining of a portion of the surface of the planar light source element. Accordingly, it has been generally considered difficult to realize the planar light source element of a structure without light diffusion film employed.
In contrast thereto, as the liquid crystal display devices have come to be largely employed, demands have been correspondingly increasing for reduction in thickness and weight, and this is particularly true of the liquid crystal display devices for use with such portable equipments as, for example, mobile phones, PDAs and video cameras. In view of this trend, as a method of realizing the reduction in thickness and weight of the planar light source element, U.S. Pat. No. 5,396,350 and the Japanese Laid-open Patent Publication No. 11-144515, for example, suggest to add the function of a prism sheet for controlling the angular distribution of light emission to the light guide plate, and this method has proven effective to a certain extent although the resultant planar light source element requires an extra use of a light diffusion film for diffusing the incoming rays of light.
Also, the Japanese Laid-open Patent Publication No. 2001-338507, for example, discloses lamination of a resin which contains a particulate matter on the outermost layer of the planar light source element, or formation of minute surface indentations on the outermost layer so as to provide the planar light source element with a light diffusing functionality. According to this known technique, the number of component parts can be reduced because it is possible for the planar light source element to be added with a function afforded by the prism sheet or the light diffusion film. However, it has been found difficult to essentially accomplish reduction in thickness because refraction and scattering of light relying on the surface contour of the planar light source element are utilized.
On the other hand, the Japanese Laid-open Patent Publications No. 2-221925 and No. 4-145485, for example, disclose addition of particulate matter to the light guide plate so that the light guide plate can have a function of the light diffusion film. However, the planar light source element disclosed therein has a problem in that the prism sheet must necessarily be employed in order to achieve the uniformity of the planar light distribution of light emission and the desired light emitting angle.